The magnetic phase transitions and magnetocaloric effect in MnNi1−xCoxGe alloys

The magnetic phase transitions and the magnetocaloric effects in MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys were investigated. The substitution of Co for Ni in the MnNiGe antiferromagnet results in the metamagnetic transitions from antiferromagnetic to ferromagnetic state, which associates with very small thermal and magnetic hystereses. Positive and negative values of magnetic entropy changes are exhibited around the metamagnetic transition temperature and Curie temperature, respectively. The relatively large refrigerant capacity in low magnetic field along with the good reversibility suggest that MnNi_1−xCo_xGe (x=0.38 and 0.40) alloys are potential candidates for magnetic refrigeration.     

Coexistence of superconductivity and magnetism in the La0.87−xLnxSr0.13FeAsO (Ln=Sm, Gd, Dy) system

The interplay between the superconducting phase and spin density wave order phase was studied. We report the magnetic and superconducting properties of the hole-doped FeAs-based superconducting compound La_0.87−xLn_xSr_0.13FeAsO (Ln=Sm, Gd, Dy; 0≤x≤0.06). Both resistivity and magnetic susceptibility measurements show that the superconducting transition temperature decreases with increase in composition of magnetic ions. The hysteresis loop of the La_0.87−xLn_xSr_0.13FeAsO sample shows a superconducting hysteresis in addition to a paramagnetic background. The experiment demonstrates that the magnetism and superconductivity coexist in hole-doped FeAs-based superconducting compounds. Among these three magnetic rare-earth elements, the influence of Dy³⁺ doping on superconductivity is more evident than that of Gd³⁺ doping, while the influence of Sm³⁺ doping is the weakest. The trend is consistent with the variation of the lattice parameter along c-axis.     

Enhanced ferroelectric, magnetic and magnetoelectric properties of Bi1−xCaxFe1−xTixO3 solid solutions

We report on the enhanced electromechanical, magnetic and magnetoelectric properties of Bi_1−xCa_xFe_1−xTi_xO₃ solid solutions. The crystal structure of the x≈0.25 compounds are close to the rhombohedral-orthorhombic phase boundary, and the solid solutions are characterized by increased electromechanical properties due to the polarization extension near the polar-nonpolar border. The homogenous weakly ferromagnetic state is established at x>0.15 doping. The chemical doping shifts the magnetic transition close to room temperature, thus enlarging the magnetic susceptibility of the compounds. The solid solutions at the morphotropic phase boundary exhibit a nearly twofold increase in piezoelectric response, whereas the magnetoelectric coupling shows five times enhancement in comparison with the parent bismuth ferrite.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Role of Mn doping for obtaining of hexagonal phase in Ba0.98Zn0.02TiO3 ceramics

This paper reports the observation of hexagonal phase of barium titanate by Mn doping and its effect on dielectric and magnetic properties. Ceramic samples of Ba_0.98Zn_0.02Ti_1−xMn_xO₃ (where, x= 0.04, 0.06 and 0.08) were prepared by traditional solid-state reaction route. The hexagonal phase is stabilized in the composition Ba_0.98Zn_0.02Ti_0.92Mn_0.08O₃ and a very feeble M–H loop is also observed in that composition. This induced magnetism is expected due to the exchange interactions between magnetic polarons formed by oxygen vacancies with Mn ions. The dielectric constant as well as the ferroelectric to paraelectric transition temperature is systematically decreased with increasing of Mn doping concentration. Further to that, the temperature dependent dielectric constant curve is also broadened at transition temperature with increasing of Mn concentration. However, the ferroelectric to paraelectric transition temperature is well above room temperature.     

Low-field magnetic properties of LaMnO3+δ with 0.065≤δ≤0.154

In a weak magnetic field LaMnO_3+δ exhibits at δ=0.065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, T_C, a mixed (spin-glass and FM) phase followed by a frustrated FM phase at δ between 0.100 and 0.154. The same behavior is observed in La_1−xCa_xMnO₃ with x between 0 and 0.3. This can be understood by the similar variation of the Mn⁴⁺ concentration, c between ≈0.13 and 0.34, in both materials when x or δ is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of T_C(c) and the magnetic susceptibility, χ(c), as well as in the critical behavior of χ(T) near T_C. These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO_3+δ than in La_1−xCa_xMnO₃.     

Synthesis, structural and physical properties of new Terbium containing Tb-Hg-Sr-Ca-Cu-O superconducting system

Samples with various nominal compositions in the Tb-Hg-Sr-Ca-Cu-O system were prepared and studied by EDX, powder X-ray diffraction including the Rietveld refinement, electrical resistivity, magnetic susceptibility and thermoelectric power measurements. EDX and powder X-ray diffraction studies showed that Tb is required for the stabilization of the 1212, (Hg_1−yTb_y)Sr₂TbCu₂O_6+δ; y≈0.5 phase. Electrical resistivity and magnetic susceptibility measurements indicated that substitution of Tb by Ca is necessary to induce superconductivity in the 1212, (Hg_0.5Tb_0.5)Sr₂(Tb_1−xCa_x)Cu₂O_6+δ samples. The Rietveld refinements of the X-ray data of two samples with x=0.0 and 0.5 were carried out on the basis of tetragonal symmetry (space group P4/mmm) and the results indicated that the phase with x=0.5 has less puckered Cu-O planes than the Ca-free (Hg_0.5Tb_0.5)Sr₂TbCu₂O_6+δ phase. Syperconductivity is observed only for samples with x>0.2 and T_c increases with increasing Ca content, x. The results of thermoelectric power measurements suggest that the samples with x<0.8 are located in the underdoped region and the x=0.8 sample is optimally doped and exhibits the highest T_c of 88 K.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Synthesis, structure and magnetic properties in the Nd2O3-Gd2O3 mixed system synthesized at 1200 °C

Phase relation studies in the Gd₂O₃-Nd₂O₃ system have been performed on (Gd_1−xNd_x)₂O₃ samples (0?x?1) with the purpose of performing a systematic study of the composition effects on their structural and magnetic properties. All the samples were synthesized by calcination of the related oxalates at 1200 °C in order to ensure the complete decomposition of the oxalates. Five phase regions, namely an A-type hexagonal, a B-type monoclinic, a C-type cubic solid solution and two biphasic mixtures of the former three phase fields were detected in this system. The magnetic susceptibility measurements showed the presence of antiferromagnetic interactions in all samples. The Curie-Weiss temperature shows a nonlinear dependence on concentration. Deduced effective magnetic moments are close to the free ion values.     

Structural and superconductivity study on α-FeSex

We have successfully synthesized the α-FeSe_x binary tetragonal superconductors with nominal composition of FeSe_x (x=0.6-1.0) via conventional solid state reactions between Fe and Se sealed in quartz tubes. Fe and β-FeSe are the most commonly seen impurities in this binary system. A low-temperature annealing at 400 °C is found to be crucial to remove β-FeSe, which is the thermodynamic stable phase with hexagonal symmetry. For all the samples of FeSe_x, superconductivity is confirmed by magnetic measurements as well as resistivity measurements with their T_c at around 8 K. We noticed that their T_c does not vary with the different nominal Se amount. High-resolution synchrotron X-ray diffraction analysis revealed that the unit cell parameters of all these samples do not change within the error range, and their structure only tolerate the same very small amount of Se deficiency. Based on this study, we concluded that the α-FeSe_x superconductor only exist in a very narrow deficiency range.     

Selected properties of EuCo2(Si1−xGex)2 alloys

EuCo₂(Si_1−xGe_x)₂, x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1 samples were synthesised by induction melting followed by annealing at 900 °C and rapid quenching. X-ray powder diffraction and Auger electron spectroscopy studies revealed that solid solutions are formed only for x?0.2 and x?0.7. Magnetic susceptibility investigations for the solid solutions revealed a dominant divalent europium valence state in the germanium-rich samples and a dominant trivalent europium component in the silicon-rich samples. In the germanium-rich samples, a long-range antiferromagnetic ordering was observed. In all samples studied, additional magnetic transitions at various temperatures were detected, which could be attributed to small clusters containing different europium chemical surrounding from that in the predominant phase.     

Effect of K doping on the physical properties of La0.65Ca0.35−xKxMnO3 (0?x?0.2) perovskite manganites

The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La_0.65Ca_0.35−xK_xMnO₃ (0?x?0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La_0.65Ca_0.35MnO₃ is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature T_C of 248 K. X-ray diffraction analysis using the Rietveld refinement show that all our synthesized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for x?0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La_0.65Ca_0.2K_0.15MnO₃ sample exhibits both phases with different proportions. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. Potassium doping leads to an enhancement in the strength of the ferromagnetic double-exchange interaction between Mn ions, and makes the system ferromagnetic at room temperature. Arrott plots show that all our samples exhibit a second-order magnetic-phase transition. The value of the critical exponent, associated with the spontaneous magnetization, decreases from 0.37 for x=0.05 to 0.3 for x=0.2. A large magnetocaloric effect (MCE) has been observed in all samples, the value of the maximum entropy change, |ΔS_m|_max, increases from 1.8 J/kg K for x=0.05 to 3.18 J/kg K for x=0.2 under a magnetic field change of 2 T. For x=0.15, the temperature dependence of |ΔS_m| presents two maxima which may arise from structural inhomogeneity.     

Optical properties and mechanical characteristics of transparent nanostructured Zn1−xMnxO thin films

Uniform and transparent thin films of Zn_1−xMn_xO (0?x?0.10) were fabricated by a sol-gel spin coating method. XRD results indicated the hexagonal structure of ZnO as the primary phase at all concentrations (x) of Mn. However, at x?0.035, Mn₃O₄ (tetragonal) is observed as the secondary phase, which was confirmed by selected-area electron diffraction patterns. SEM and TEM results showed a tendency of grains to arrange into wire-shaped morphologies, leading to elongated needle-like structures at high Mn addition. Increasing Mn content in the range 0?x?0.10 led to quenching of photoluminescence, increase in the band gap (E_g) from 3.27 to 3.33 eV, and increase in film thickness, refractive index and extinction coefficient of Zn_1−xMn_xO thin films. The residual stress evaluated was compressive in all cases and found to increase by an order of magnitude with addition of Mn. Furthermore, an overall increase in microhardness and yield strength of Zn_1−xMn_xO thin films at higher Mn concentrations is attributed to change in microstructures, presence of secondary phase and increase in film thickness.     

Superconductivity in Sr and Co co-doped PrFeAsO

Polycrystalline Sr and Co co-doped superconducting oxypnictides Pr_1−ySr_yFe_1−xCo_xAsO were synthesized by solid state reaction method, and superconductivity was investigated by measuring resistivity, magnetic susceptibility and thermopower. In the PrFe_1−xCo_xAsO samples with only Co doping, T_c reaches a maximum of 16 K at x=0.075-0.1, and the variation of T_c with Co content (x) is dome-shaped, consistent with the other Co-doped 1 1 1 1 phase systems such as LaFe_1−xCo_xAsO and SmFe_1−xCo_xAsO. In the Co and Sr co-doped Pr_0.8Sr_0.2Fe_1−xCo_xAsO system, T_c reaches a maximum of about 16 K at x=0.15, and the T_c‐x dome shifts to higher Co content in the phase diagram of T_c versus Co content (x). Our result indicates that Sr doping effectively induces hole-type charge carriers in the PrFeAsO system and the disorder caused by Sr and Co dopants has little effect on T_c. The thermopower data confirms that Sr dopant is hole-type and Co dopant is electron-type. The shift of T_c‐x dome to higher Co content caused by Sr doping implies that the Sr dopant can compensate for the Co doping effect. The results suggest that there could be a universal dependence of T_c on charge carrier density in both hole-type and electron-type oxypnictide superconductors.     

Magnetic and related properties of the solid solution CeCuxGa4−x

Physical properties of polycrystalline samples of CeCu_xGa_4−x (x = 0.2–1.4), crystallizing in the tetragonal BaAl₄-type structure (space group I 4/mmm), were studied by means of X-ray powder diffraction, magnetization, specific heat, electrical resistivity and magnetoresistivity measurements in wide temperature and magnetic fields ranges. The unit-cell volume of the system was found to decrease with increasing x (in total by about 4%) but the magnetic moments of Ce³⁺ ions remain localized in the whole x-range studied. The alloys exhibit ferromagnetic order at low temperatures, which manifests itself as distinct and relatively sharp anomalies in all the temperature characteristics measured. The ordering temperature decreases with increasing the Cu content from 5.5(1) K for x = 0.2 down to 1.35(5) K for x = 1.4, and the electrical transport properties of the system show some features characteristic of Kondo lattices.     

Characterization of multiferroic LiTMxCu2−xO2 (TM=Ni and Zn) single crystals

We investigate a series of single crystalline samples of LiT_MxCu_2−xO₂ (T_M=Ni and Zn) grown by floating-zone technique. As-grown crystals showed a clear X-ray diffraction pattern of pure(single) phase. These crystals exhibit varied magnetic and electrical properties owing to successful incorporation of Ni and Zn dopants.     

Effect of Gd doping on structural, electrical and magnetic properties of BiFeO3 electroceramic

Room temperature multiferroic electroceramics of Gd doped BiFeO₃ monophasic materials have been synthesized adopting a slow step sintering schedule. Incorporation of Gd nucleates the development of orthorhombic grain growth habit without the appearance of any significant impurity phases with respect to original rhombohedral (R3c) phase of un-doped BiFeO₃. It is observed that, the materials showed room temperature enhanced electric polarization as well as ferromagnetism when rare earth ions like Gd doping is critically optimized (x=0.15) in the composition formula of Bi_1+2xGd_2x/2Fe_1−2xO₃. We believe that magnetic moment of Gd⁺³ ions in Gd doped BiFeO₃ tends to align in the same direction with respect to ferromagnetic component associated with the iron sub lattice. The dielectric constant as well as loss factor shows strong dispersion at lower frequencies and the value of leakage current is greatly suppressed with the increase in concentration of x in the above composition. Addition of excess bismuth and Gd (x=0.1 and 0.15) caused structural transformation as well as compensated bismuth loss during high temperature sintering. Doping of Gd in BiFeO₃ also suppresses spiral spin modulation structure, which can change Fe-O-Fe bond angle or spin order resulting in enhanced ferromagnetic property.